Compliant bump technology

ABSTRACT

The specification describes an interconnection technique using compliant metal coated photodefined polymer bumps for mounting and interconnecting component assemblies on substrates such as glass, printed wiring boards, etc. The polymer chosen for the bump structure has a relatively low T g  and the polymer bump is metallized in a way that substantially encapsulates the polymer.

FIELD OF THE INVENTION

This invention relates to interconnection of electrical and opticalcomponents on support substrates. More specifically it relates to animproved bump technology for flat panel displays (FPD), multichipmodules (MCM), printed wiring board (PWB) interconnections, and thelike.

BACKGROUND OF THE INVENTION

Mounting and electrically connecting electrical and optical integratedcircuit packages, and optical components such as lasers and LEDs hasspawned a variety of interconnection technologies aimed at increasingthe interconnect density, increasing reliability, and decreasingassembly costs.

Bump technology has developed along two paths, one for gold bumps usedin displays, and one for solder bumps or solder balls used typically insolder assemblies, such as PWBs and MCMs. In current bump technologyboth gold bumps and solder bumps/balls are formed typically by plating,In some processes evaporation techniques have been used for formingsolder bumps, since solder evaporates at a conveniently low temperature.In addition to gold and solder, other metals such as aluminum, nickel,copper, titanium, cobalt, tantalum, the platinum group metals, andalloys thereof are all candidates for current bump technology.

Solder bump/balls are typically 2-6 mils in height. The typicalattachment technology is Surface Mount Technology (SMT). The SMT processis usually performed by stencil printing a eutectic solder paste on thePWB bond pads, placing the chip on the PWB with solder bumps/ballsaligned to the solder paste coated bonding pads, and completing theattachment by solder reflow.

Gold bump chips are typically of the order of 20 microns high, and areused in TAB packages, and in chip-on-glass (COG) technology. Early COGtechniques resembled the conventional lead-frame technique where thechip substrate was bonded face up to the glass support and the bondingpads on the chip were connected to the circuit pattern on the glassusing wire bonds. Current COG techniques follow a flip chip approach(FCOG) with the bonding pads on the chip aligned with bonding sites onthe glass. These interconnection processes typically use gold bumps onthe chip, and adhesives, typically Anisotropic Conductive Films (ACF) toadhere the gold bumps to the bonding sites on the glass. Anisotropicconductive films have proven reliable and cost effective. Details on ACFmaterials and their use is given by Hisashi Ando et al, AnisotropicConductive Film, available from Sony Chemical Corporation.

A currently used FCOG process using ACF material involves gold bumpsthat have very flat bump tops. Typically the bumps are formed by platingthe chip surface and lithographically defining the bump sites. Theassembly sequence is as follows. The ACF is tacked to the chip bondingsite on the glass substrate. The chip is picked up by the automatedassembly equipment, aligned to the bond site, and heated to theappropriate bonding temperature. The chip is then placed, pushed throughthe ACF, and held in place with an appropriate bonding force applieduntil the adhesive reaches the desired state of cure. During the bondingprocess the conducting particles within the ACF are trapped between thegold bump and the bond pad, and compressed to deform the particles.Particles typically 7-8 microns in diameter are compressed to 3-5microns in height. The particles form a monolayer between the chip bumpsand the bonding sites so that a large number of conductive paths areformed in the z-direction, while remaining spaced and thereforeinsulated from one another in the x-y directions.

For these techniques to be effective, the separation between theelements being bonded must be highly uniform in order to develop aconsistent level of bonding pressure at each bonding site. To obtainuniform spacing requires precise control over the thickness of thebonding pads, the thickness uniformity of the glass and chip substrate,and the vertical (thickness) dimension of the bumps. Variations on theorder of one to two microns are sufficient to cause inadequatecompression of the ACF material and result in defects in the bondingoperation.

Thickness uniformity can be improved and the reliability of ACFtechniques can be increased using photodefinable polymers applied to thechip substrate. These materials are liquid as applied and thus tend tocure with a very planar surface. After the polymer bumps are definedlithographically they are coated with a conductive film. One suchapproach is referred to by Candice Hellen Brown, Flat Panel Display fromComponent to Substrate, ISHM '93 Proceedings, pp. 249-253. Such polymerbumps are compliant. This addresses an important problem because thez-axis compliance tends to compensate for the spacing variations justdescribed. Also the x- and y-axis compliance improves the temperaturecycling reliability in solder assembly processes. However, with thecurrent state of the art the solder bump/ball interconnection cannotwithstand the strains associated with the thermal mismatch betweentypical chip and PWB materials.

While the suggestion of using a compliant polymer bump should advancethe art toward the elimination of the problems just outlined, aneffective photodefinable polymer bump technology does not exist in theprior art.

Statement of the Invention

We have developed a compliant polymer bump technology in which the bumpsare compressible and are easily deformed when the component chip orassembly is pressed against the support substrate. The requiredobjective is for the bumps to form a gap for the conductive monolayerand the size of the gap is fixed by the properties of the ACF material.The gap is therefore the same for each bump site. Since the bumps arecompressible, each bump is able to accommodate that common spacing. Thecompliant bumps according to our invention are photodefinable polymerbodies coated with a conductive metal layer as more specificallydescribed below. The polymer used in our invention is a knownphotodefinable dielectric material comprising from twenty to sixtyweight percent of triazine and from one to ten weight percent ofsiloxane-caprolactone copolymer. This material has a glass transitiontemperature T_(g) of approximately 180° C. Since a typical bondingprocess requires temperatures of the order of 220°C.-240° C., it mightbe expected that this material would not perform adequately as a polymerbump material. However, it has proven highly successful in practice. Thereason why a material with such a low T_(g) survives in this applicationis not known. We theorize that it is helpful to use a bump structure inwhich the polymer bump is encapsulated in the conductive metal. Such anarrangement would be expected to aid in preserving the bump geometryduring processing at elevated temperatures. While we experienced successwith the specific polymer material described, other equivalent materialsmay be used as well. Equivalent properties would include a realtivelylow (less than 200° C.) T_(g).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic representations of the bonding operationusing standard bump arrays;

FIG. 3 is a schematic representation similar to that of FIGS. 1 and 2showing the bonding operation using a compliant polymer bump array; and

FIG. 4 is a front section of a bump structure according to theinvention.

DETAILED DESCRIPTION

Referring to FIG. 1, a portion of a substrate is shown at 11 with anarray of contact pads 12 formed e.g. by standard printed circuittechniques. The device chip or assembly 13 carries an array of bumps 14which may be gold bumps or solder bumps as are standard in the art. Bump14b is deliberately shown with a smaller profile, i.e. bump height, thanbump 14a. The difference is exaggerated for illustrative purposes butrepresents in a schematic way the reality of variations that occur innormal processing. The ACF film is shown at 15 with conductive particlesat 16. In a typical interconnection process, e.g. flip chip on glass(FCOG), the glass substrate is of the order of 1-3 mm thick and thebonding areas 12 are portions of a metallization pattern, e.g. indiumtin oxide, or aluminum. The pitch of these pads in current technology isof the order of 50-200 μm and the spacing between pads may be 20-50 μm.The bumps 14 in this illustration are gold bumps with dimensionscorresponding to the contact pad array of the substrate in the x-yplane, and with a height (z-dimension) of typically 15-30 μm. The devicepackage is square or rectangular with sides measuring typically in the2-50 mm range.

Standard ACF materials are typically epoxies, and the conductiveparticles are typically gold-plated epoxy resin particles. The films aregenerally 10-40 μm thick and the conductive particles are approximately5-10 μm in diameter.

As seen in FIG. 2, after the bonding process has occurred, with thecomponent assembly 13 and the substrate 11 thermocompressively bonded,the desired monolayer of conductive particles 16 are properly trappedunder bump 14a, to give a conductive path between the package and thesubstrate. However, the ACF region under bump 14b is not sufficientlycompressed to form a reliable conductive particle monolayer, i.e. thereis no conductive particle continuity between the bump and the substratepad. This region remains the same as the regions between the bumps andpads, and is insulating.

In FIG. 3 the bumps comprise a metal coated polymer according to theinvention. The polymer bumps 14 are coated with metal layer 31. Afterbonding, it is seen that the bumps are partially compressed. Thiscompression allows all of the bumps to compress the conductive particlesin the ACF material sufficiently to form the conductive particle bridgebetween the substrate and component package. The conductive particles,typically of the order of 7 microns in their original shape (FIG. 1) arecompressed to about 3 microns. Meanwhile the bumps themselves compresstypically by 2-3 microns. In FIG. 3 it is evident that the bump with thelowest height in the z-direction, bump designated 14b, adequatelycompresses the ACF material to form a conductive particle path betweenpad 12 and the component assembly 13. The higher bump 14a undergoessomewhat more compression and deformation, as shown, to form theconductive bond at its site. The compliancy of the bumps allows allbumps in the array to self adjust to fill the space required but stillimpart sufficient compression to trap conductive particles beneath eachbump. To achieve this it is recommended that the polymer material usedto form the bumps be less compliant than the epoxy material of theconductive particles. One method to achieve the desired compliancerelationship is to have the T_(g) of the epoxy relatively sharp, i.e.sharper than the T_(g) of the polymer, and lower in temperature than theT_(g) of the polymer.

It will be evident to those skilled in the art that the mechanicalproperties of the bump are critical to a successful bonding operation.The first requirement is that the bump material must withstand thetemperatures of the bonding operation, i.e. 220°-240° C. At the bondingtemperature the material should provide the desired resiliency whilestill retaining the original bump shape, size and position. It must becompliant, but retain sufficient firmness to support the hydrodynamicpressure required to compress the ACF material and retain the conductiveparticle monolayer as described above. These considerations point to amaterial with a glass transition temperature T_(g) of 220° C., theminimum bonding temperature, or above.

We have discovered that contrary to expectations, a polymer materialwith a T_(g) substantially, i.e. at least 20° C., below the bondingtemperature performs exceptionally well in this process. We have notbeen able to establish the reason for this performance. However wepostulate that the success is characteristic of materials withrelatively low T_(g) values, and especially where the T_(g) of the epoxyparticles is below the T_(g) of the polymer bump material. It may alsobe attributable in part to the structure of the bump. In any case wehave demonstrated successful bonding processes where other bumpmaterials and bonding approaches have not been successful.

The structure of the bump is shown in FIG. 4. The substrate of thecomponent assembly is shown at 41. The substrate 41 is typically asemiconductor chip. One or more chips, and multiple bonding sites,comprise the total package but only one bonding site and bumpinterconnect is shown for simplicity. The chip 41 carries a conventionalmetallization pattern of which just bonding pad 42 is shown. The polymerbump 43 is shown adjacent the bonding pad 42 with metallization layer 43formed so as to contact the bonding pad 42 and to cover the exposedsurface of the polymer bump 43. While the metallization layer is showncovering the bonding pad 42 in FIG. 4, it is sufficient that it contacta portion of the pad. However, with the polymer bump we prefer that themetal covers the entire exposed surface of the bump. The mechanicalproperties of the metallization layer, primarily the thickness andshape, contribute to the overall mechanical properties of the bump. Theshape of the metal layer is essentially determined by the portion of thebump that is covered. We theorize that the combination of a low T_(g)material and the metallization encapsulant, i.e. the metal covering theentire free surfaces, of the polymer bump are responsible for thesuccess of our process. However, the most unique feature is the use of aphotodefinable low T_(g) material, an example of which we describebelow. The photodefinable feature simplifies the bonding process andreduces the cost. The polymer bump represented by 43 in FIG. 4 wasformed by coating the surface of the component package 41 with a polymerlayer, by spray coating, or other appropriate technique, then maskingthe polymer layer with a photomask, exposing the mask to UV radiation,and removing the unexposed regions of the polymer layer by means of adeveloping process or using a developing solution.

The photodefinable polymer successfully used in our process is describedand claimed in U.S. Pat. No. 5,326,671, which is incorporated herein byreference. The polymer is a photodefinable triazine-based mixtureincluding a photosensitive acrylate moiety. It comprises from twenty tosixty percent by weight of triazine and from one to ten percent byweight of siloxane-caprolactone copolymer. It may also include up totwenty percent by weight of novolak epoxy acrylate to improve thephotodefinable properties. Additionally the mixture may optionallycomprise one or more of the following: two to eight percent by weight ofbis-phenol-A diglycidyl ether monoepoxyacrylate, zero to twenty percentby weight of carboxyl-terminated butadiene nitrile rubber, two to sixpercent of N-vinylpyyrolidone, one to ten percent oftrimethylolpropanetriacrylate, zero to five percentglycidoxypropyltrimethoxysilane, 0.05 to five weight percentphotoinitiator, zero to two percent pigment, 0.1 to one percentsurfactant, zero to 0.3 percent copper benzoylacetonate, and thirty tofifty percent solvent.

In our process the triazine composition just defined is spray coated onsubstrate 41 to the desired thickness then baked at 50 degrees C for twohours. Using the technique of the invention we have successfullyproduced bumps from a few microns to 8 mils in thickness. The portion 43of the layer (remaining layer not shown in FIG. 4) is exposed to actinicradiation i.e. UV radiation at 365 nm with a power of twenty to fortymilliwatts per square centimeter. Exposure to the actinic lightcrosslinks and cures the polymer making it insoluble in the developersolution. The layer is developed by spraying on a suitable developer,e.g. butyl butyrate, thereby removing the unexposed regions and leavingthe array of polymer bumps represented by 43. The material of theseparticle bumps has an average T_(g) of less than 200 degrees C.,typically approximately 180 degrees C.

It is characteristic of this photodefined material to have slopedsidewalls, as shown at 45 in FIG. 4. This is due to the materialproperties and the isotropic nature of the dissolution process duringdevelopment. The ratio of solubility between the exposed and unexposedregions of the polymer layer is finite, so that some of the exposedsurface regions of the layer dissolve slightly while the solventdescends through the thickness of the unexposed regions. The sidewalltaper is slight but is helpful in promoting sidewall coverage when thebump is metallized.

The metallization layer 44 is then applied by sputtering, evaporation,or other appropriate technique. We successfully used sputtering toproduce the structure of FIG. 4. The thickness of layer 44 is preferablyin the range 1-8 microns. The material of layer 44 may be selected froma variety of metals as stated previously. We sputtered a layer oftitanium, 1000-3000 Angstroms in thickness, followed by an alloy layerof Ti-Pd 10-200 Angstroms thick, followed by approximately 1-6 micronsof copper. Optionally we followed the foregoing three layers with afinal coating of 2000-6000 Angstroms of Ni, and 1000-3000 Angstroms ofgold.

While we have illustrated and described a compliant polymer bumpinterconnect technology in which the polymer bumps are essentiallycompletely encapsulated by metal, variations of this structure maypermit partial encapsulation while still substantially achieving theadvantages we describe. Accordingly we define our invention in itsbroadest terms as requiring a metallization layer that encapsulates asubstantial portion of the polymer bump. More specifically we define astructure in which the top surface of the polymer bump is covered with ametal layer, with portions of the metal layer extending down at leasttwo sides of the bump. A structure can be envisioned in which thepolymer bump is rectangular, and metal encapsulating the long sides ofthe rectangular structure would substantially meet the objective ofmechanically containing the polymer bump during bonding.

The bonding operation is conventional and involves the application ofheat to the surfaces being bonded, while pressing the surfaces together.As suggested earlier the ACF bonding temperatures are typically above200 degrees C. The pressure depends upon the ACF material and usually isabove 0.5 tons per square inch of bump area, typically about 5 tons persquare inch of bump area.

The metal coated polymer material is known to withstand typical SMTsolder reflow processes so that solder attachment for the polymer bumptechnology and those skilled in the art will recognize the utility ofthe improved bump technology of this invention to typical prior artsolder bump techniques.

The interconnect technique of the invention can be used to advantage ina wide variety of applications. Most of these will involve electronic orphotonic integrated circuits and electrooptic devices. For the purposeof definition the term electronic package is intended to covergenerically all such integrated circuit devices, electrooptic devices,and related electrical products.

Various additional modifications of this invention will occur to thoseskilled in the art. All deviations from the specific teachings of thisspecification that basically rely on the principles and theirequivalents through which the art has been advanced are properlyconsidered within the scope of the invention as described and claimed.

We claim:
 1. Method for bonding two elements together, said two elementsconsisting of an electrical or photonic component and a substrate forsupporting said component, the method comprising the steps of:a. forminga plurality of bonding pad on the first of said elements, b. depositinga layer of photodefinable polymer on the surface of said second element,said photodefinable polymer having a glass transition temperature T_(g),c. masking said layer of photodefinable polymer with a mask having aplurality of actinic light transparent features corresponding to aplurality of bumps, d. exposing said masked photodefinable layer toactinic light, e. developing said photodefinable layer to produce aplurality of bump features on the surface of said second element, eachof said plurality of bump features having a to p surface and sidewallsurfaces exposed by the developing step e., f. coating the top surfaceand the sidewall surfaces of each of said bumps with a metal coating sothat the polymer bump is substantially completely encased in said metalcoating, g. positioning the said plurality of bumps in registration withsaid plurality of bonding pads, h. providing a layer of anisotropicconductive film between the plurality of bonding pads and the pluralityof bumps, i. urging said first and second elements together, j. heatingsaid first and second elements to a bonding temperature, while applyingpressure to said elements to bond said plurality of bumps to saidplurality of bonding pads.
 2. The method of claim 1 in which the glasstransition temperature of said polymer is less than the said bondingtemperature.
 3. The method of claim 2 in which the anisotropicconductive film consists of a polymer material with a glass transitiontemperature less than the glass transition temperature of said polymercomprising said bumps.
 4. The method of claim 2 in which the saidbonding temperature is in the range 220°-240° C. and the said glasstransition temperature of the polymer of said bumps is less than 200° C.5. The method of claim 2 in which the said metal coating comprisescopper.
 6. The method of claim 2 in which the thickness of saidphotodefinable layer is in the range of 2-400 microns.
 7. The method ofclaim 2 in which the sidewalls of the bumps are sloped.
 8. The method ofclaim 2 in which the said polymer is formed from a mixture comprisingtwenty to sixty weight percent of triazine and from one to ten weightpercent of siloxane-caprolactone copolymer.
 9. The method of claim 8 inwhich the said mixture further includes up to ten weight percent ofnovolak epoxy acrylate.
 10. The method of claim 9 in which the saidmixture includes one or more of the following:two to eight percent byweight of bis-phenol-A diglycidyl ether monoepoxyacrylate, zero totwenty percent by weight of carboxyl-terminated butadiene nitrilerubber, two to six percent of N-vinylpyyrolidone, one to ten percent oftrimethylolpropanetriacrylate, zero to five percentglycidoxypropyltrimethoxysilane, 0.05 to five weight percentphotoinitiator, zero to two percent pigment, 0.1 to one percentsurfactant, zero to 0.3 percent copper benzoylacetonate, and thirty tofifty percent solvent.
 11. The method of claim 2 in which the metalcoating is selected from the group consisting of Al, Ni, Cu, Ti, Co, Ta,Pt, Pd, and alloys thereof.